1. Field of the Invention
The present invention generally relates to a circuit for an optical Wavelength-Division Multiplexing (WDM) system, and in particular to an optical circuit for extracting an optical signal on a predetermined wavelength from optical WDM signals.
2. Description of the Related Art
The advent of optical-fiber amplifiers causes losses due to optical fibers and optical elements to be easily compensated for, stimulating the research and development of Wavelength-Division Multiplexing which is the current favorite multiplexing technology for optical communication networks. In such an optical WDM communications network, a wavelength Add/Drop multiplexer abbreviated by ADM is needed to realize a multi-point network which can provide communications between a plurality of locations. The ADM is used to drop an optical signal on a predetermined wavelength and to add a new data stream to the same wavelength at the location thereof.
A conventional ADM has been proposed in Japanese Patent Application Unexamined Publication No. 8-248460. The conventional ADM inserted on a physical fiber link is composed of an Add/Drop optical filter.
In general, such an Add/Drop filter is composed of a fixed-wavelength filter for each wavelength. Therefore, in the case of a change in network configuration, the conventional ADM cannot easily respond to such a change with flexibility.
Further, in cases where the wavelength of a transmission optical signal changes over time, there is a possibility that the ADM becomes dysfunctional and puts communications out of operation.
An object of the present invention is to provide an optical circuit that can increase flexibility in optical signal add/drop operation.
Another object of the present invention is to provide an optical circuit that can provide add/drop control with reliability even when the network configuration is changed and/or the wavelength of an optical signal varies with time.
According to an aspect of the present invention, an optical circuit receives optical WDM (wavelength-division multiplexing) signals at an input port. The optical circuit is comprised of a variable-wavelength optical notch filter having a notch wavelength. The variable-wavelength optical notch filter receives the optical WDM signals to allow an optical signal of the notch wavelength to be reflected and another optical signal of a wavelength different from the notch wavelength to pass through. A control signal is detected from the optical WDM signals, and the notch wavelength of the variable-wavelength optical notch filter is controlled based on the control signal. The notch wavelength may be determined according to the control signal. Therefore, the variable-wavelength optical notch filter can be set to a desired notch wavelength to allow easy changes in wavelength of a dropped optical signal.
The circuit further includes a level detector for detecting a signal level of a reflected optical signal reflected by the variable-wavelength optical notch filter. The notch wavelength is controlled such that the signal level of the reflected optical signal is maximized. Therefore, the variable-wavelength optical notch filter can be optimally adjusted at all times.
According to another aspect of the present invention, an optical circuit receives optical WDM (wavelength-division multiplexing) signals at an input port. The optical circuit is comprised of a variable-wavelength optical notch filter having a notch wavelength. The variable-wavelength optical notch filter receives the optical WDM signals to allow an optical signal of the notch wavelength to be reflected and another optical signal of a wavelength different from the notch wavelength to pass through. The optical circuit further includes a level detector for detecting a signal level of a reflected optical signal reflected by the variable-wavelength optical notch filter. The notch wavelength of the variable-wavelength optical notch filter is controlled such that the signal level of the reflected optical signal is maximized. Therefore, the variable-wavelength optical notch filter can be optimally adjusted at all times.